1. Field of the Invention
This invention relates to the purification of compositions containing rare-earth elements. More particularly, it relates to processes for removing trace quantities of radioactive impurities from compositions containing rare-earth elements.
2. Discussion Relative to the Prior Art
Phosphor materials comprising a rare-earth element as a host ion are well-known in the art. These phosphors are typically activated by the presence of minor amounts of other rare-earth elements. The rare earths are also referred to as lanthanides and are characterized by having an atomic number from 57-71 inclusive. Two rare earths, namely, lanthanum and gadolinium, are particularly useful as host ions in phosphor materials. An important use of these lanthanum and gadolinium rare-earth phosphor materials is in X-ray screens. A layer of the phosphor material serves to convert an X-ray image pattern into an image pattern which can be recorded on a photographic film positioned adjacent the X-ray screen. One particular phosphor material is lanthanum oxysulfide activated with terbium.
It has been observed that rare-earth phosphor materials may spontaneously emit radiation which can interfere with their use in X-ray screens. Because photographic film is sensitive to the unwanted spontaneous emissions of these phosphors, using these phosphors can result in the formation of undesired exposed areas which degrade the quality of the image. These undesired exposed areas take the form of high-density spots where the film is exposed to the spontaneously emitted radiation. The principal source of this spontaneously emitted radiation is radioactive impurities in the ore which is the source of the rare earth for the phosphor. The severity of the degradation is a function of the concentration of the radioactive impurities and the period of time for which the photographic film is in contact with the X-ray screen containing the impurity-laden phosphor.
Ores containing significant amounts of the rare-earth elements, such as monazite ore and bastnaesite ore, also typically contain radioactive elements such as uranium, actinium and the radioactive members of the uranium and actinium radioactive decay chains. Because uranium and most other radioactive impurities have differences in chemical properties distinct from the rare-earth elements, it is possible to separate readily most of these impurities from the rare earth. However, small amounts of radioactive impurities typically remain. The most difficult impurities to remove have proven to be actinium (227) and the members of its decay chain. This is because the chemical properties of actinium are very similar to the chemical properties of the rare-earth elements, particularly lanthanum. Therefore, chemical separation techniques which are adequate to separate other radioactive impurities, such as uranium, from the rare-earth elements, will typically not achieve satisfactory separation of actinium from the rare earths. In order to eliminate substantially the spotting problem discussed above, it has been found necessary to reduce the level of radioactive impurities in the rare-earth phosphor to less than 1 part in 10.sup.18. Prior-art methods for separating actinium from the rare earths, such as those discussed below, have proven to be inadequate to provide this level of purity.
Several methods have been devised for obtaining separations of actinium from the rare earths. In one of the earliest attempts, a material enriched in actinium was obtained by a process including fractional precipitation of lanthanum from solution, using oxalic acid. The unprecipitated material was reported to have shown an increased proportion of actinium which would, of course, mean that the precipitated rare earth would be depleted of actinium (Curie, J. Chim. Phys., vol. 27, pp. 1-8 (1930)). Another process is described in U.S. Pat. No. 2,425,573 to Soddy. In this process, the rare-earth sulfates are precipitated in the presence of phosphoric acid, leaving behind in the filtrate the unwanted thorium. Thorium is one step away from actinium in the actinium decay chain. Again, the precipitated rare earth would presumably be depleted of an unwanted impurity. While these processes and others have been useful in purifying the rare earths to a certain extent, none of these processes can economically produce rare earths which are pure enough for use as phosphors in X-ray screens. There is a continuing need for an economical and efficient process for removing the radioactive impurities from the rare-earth elements.